The present invention relates generally to an H-Bridge switching circuit that can be used in a power amplifier for a radio transmitter or a switching power supply.
It has been well known in the switching power supply industry that high efficiency systems can be designed by chopping a voltage to produce another voltage. Pulse width regulated power supplies operate at efficiencies in excess of 97%. However, the higher switching frequencies needed to operate in the AM band tend to increase switching losses. Also, it becomes difficult to turn FETs, which are commonly used in switching power supplies, on and off quickly enough when the duty cycle is small.
It has always been a goal to produce AM transmitters that will be more efficient. AM transmitters using two stages to do the modulation and the amplification of the audio signal are known and are suitable for handling transmitters up to a power of 10 kW. However, it is difficult to reach necessary efficiencies (e.g., 86%) using a separate modulator and amplifier for higher power level (e.g., 25 kW, 50 kW and 100 kW) transmitters. An amplification-modulation scheme that is much more efficient is required for the higher power levels.
One way to improve efficiency is to remove sections of the transmitter or to group different parts of the transmitter. In general, an AM transmitter comprises a power supply, modulator, power amplifier, combiner, and output network. Each of these parts has an associated energy loss that adds up to form the total system loss.
It is known to combine the modulation and the amplification into one stage to improve efficiency. To perform the modulation of the transmitters, known systems turn the amplifiers off and on. These amplifiers are added through RF combiner transformers on the output of each RF stage. This means that many amplifiers are needed to reduce the change in amplitude when one module is turned off or on. These systems prove impractical below the 10 kW power level because of the large number of amplifiers needed to apply this scheme.
A design known from U.S. Pat. No. 4,580,111 can operate fairly efficiently, however, one difficulty with this type of design is that it is only practical for high power (e.g., 10 kW and above—at, e.g., 9 kW watts and below, particularly at 5 kW and below, this circuit becomes increasingly impractical). To build this type of design, enough modules are needed so the step size in turning a module off and on will be small.